This invention relates to the digital communication art and, more particularly, to a means of, and method for resolving synchronization in an error correction encoded transmission.
Digitally encoded communication systems are well known in the data transmission art. In such systems, a data bit stream containing digitally encoded information is to be transmitted over a noisy transmission medium. If the raw information data were transmitted without being processed, there would exist a substantial probability that portions of the message would be lost due to interference from the noisy medium. Therefore, this data is normally processed to produce a transmission bit stream which, after decoding at the receiver site, provides a means to correct or minimize transmission medium induced errors. Two examples of error correction processing schemes are block and convolutional type codes.
Proper decoding of the transmission bit stream requires that the decoder be able to recognize and synchronize with a received bit stream signal. One method known to the prior art for assuring proper decoding synchronization is the use of a predetermined set of sync bits which are sent immediately prior to the information data bits. The sync bits follow a predetermined pattern, such as one of the well known Barker sequences, which optimizes the ability of the decoder to recognize and synchronize with the transmission bit stream.
A problem with the aforementioned prior art synchronization schemes is that the transmission bit stream has a minimum length equal to the number of information plus sync bits. In some systems, the number of sync bits must be substantial with respect to the number of data bits to insure the desired probability of synchronization whereby the overall message length is quite long, thus requiring a lengthy transmission time. It is, of course, desirable to reduce transmission time to an absolute minimum.
In one approach which minimizes transmission bit length, the error correction encoded data is modulo-2 added with a polynomial generated from a binary word generator. The polynomial is selected based on the nature of the correctable error encoding scheme being employed. In so doing, processing circuitry at the decoder may determine from the recovered data bit error pattern the condition of either a loss of synchronization or channel induced errors. However, a fundamental problem with this system is that the polynomial is strictly a function of the particular coding scheme being used and, thus, must be suitably updated each time a different code is employed. In addition, this scheme necessarily employs a tradeoff between the ability to detect synchronization and the ability to distinguish synchronization from medium induced errors. Thus, the system does not provide optimized means for determining the state of synchronization.
In an alternate approach, a parity bit is added at the end of each transmitted word. A parity monitor at the decoding site detects the condition of disparity. An occasional departure from a parity constraint is allowed, however a grouping which repeatedly produces a condition of disparity is used to indicate an error in synchronization. A fundamental problem with this approach is that, for many applications, the system does not indicate the out-of-sychronization condition quickly enough, nor with high enough probability to render its performance acceptable.